Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
One particular zeolite, designated ZSM-18 was first disclosed by Ciric in 1976 (See U.S. Pat. No. 3,950,496). This patent describes the synthesis of ZSM-18 from a triquat structure directing agent, 2,3,4,5,6,7,8,9-octahydro-2,2,5,5,8,8-hexamethyl-1H-benzo[1,2-c:3,4-c:5,6-c]tripyrrolium trihydroxide (triquat 1). The ZSM-18 was found to have pore openings greater than 7 Å based on the adsorption of cyclohexane and Si/Al from 5 to 15. Na+ was also employed in the form of sodium aluminate and helped balance the framework charge from within the pores. It was also noted that if the Si/Al ratio of the reaction mixture was less than 10, the resulting ZSM-18 was unstable to calcination. It wasn't until 1990 that the structure of ZSM-18 was reported by Lawton et. al., revealing 1-dimensional 12-ring pores with the expected 7 Å aperture along with a perpendicular 7-ring pore system (See SCIENCE, 247, 1319-1322 (1990). The structure also contained the first 3-rings observed in an aluminosilicate. The authors attempted to examine the role of the triquat cation in the synthesis of ZSM-18 by fitting the structure of the triquat within the 12-ring pore, finding it could only occupy very specific positions. This was interpreted as a strong templating effect by the triquat and that such bulky, multiply charged templating cations may be required to make the observed zeolite with the 3-rings.
In 1994, a new synthesis for ZSM-18 was disclosed by Schmitt (See U.S. Pat. No. 5,350,570) which used a different triquat, [(Me3N+(CH2)2)3N]*3OH−, (triquat 2). Schmitt pointed out that little had been done with ZSM-18 since 1976 because the triquat 1 was very expensive and difficult to make and because thermal decomposition during calcination often destroyed the ZSM-18 sample. The new triquat 2 also possesses the bulky 3-fold, multiply charged structure reminiscent of triquat 1, which is seen as a requirement to make ZSM-18 via a templating effect. While the thermal decomposition of ZSM-18 with triquat 2 did successfully yield stable protonic forms of ZSM-18; it is now 12 years later and still little has been done with ZSM-18. This is because while triquat 2 is more economical than triquat 1, it is stated in the '570 patent that it is derived from a fine chemical and therefore remains too expensive for convenient use. A “rational synthesis” route to ZSM-18 was also disclosed in 1994 (ZEOLITES, 14, 635-642 (1994)). This rational approach acknowledges the same difficulties enumerated in the '570 patent with respect to economical synthesis and thermal degradation of triquat 1. This paper also cites “the almost perfect fit” of the triquat 1 template in the 12-ring pore concluding that if a templating effect exists, it is present in ZSM-18. This idea is then used to select template alternatives to triquat 1, with various types of modeling leading to triquat 2 mentioned above and [(Me3N+(CH2)2)3CH]*3OH−triquat 3, which is the same as triquat 2, except with the central N replaced with C—H. Triquat 3 managed to produce ZSM-18 only in the presence of seeds while triquat 2 produced a low quality ZSM-18 directly, but a highly crystalline ZSM-18 when seeds were employed in the synthesis. Zeolite ZSM-18 is still not used or studied because of the difficulty and expense of its preparation.
Applicants' composition is obtained by calcining an as synthesized microporous composition designated UZM-22. UZM-22 compositions are prepared via the use of a simple commercially available structure directing agents, such as choline hydroxide, [HO(CH2)2NMe3]+OH−, in concert with small amounts of Sr2+, Li+, or Sr2+ and Li+ together, using the Charge Density Mismatch Approach to zeolite synthesis as disclosed in U.S. Pat. No. 7,578,993 B1.